A bacteriochlorophyll a antenna complex from purple bacteria absorbing at 963 nm

A recently isolated species of the photosynthetic purple sulfur bacteria, p rovisionally called strain 970, was investigated with respect to its antenn a function by means of various spectroscopic techniques, including fluoresc ence and pump-probe absorption difference spectroscopy. The bacterium conta ins bacteriochlorophyll a and an as yet unidentified carotenoid, perhaps 3, 4,3 ' ,4 ' -tetrahydrospirilloxanthin. It has a single antenna complex of t he LH1 type, with a Q(y) absorption band situated at the unusually long wav elength of 963 nm at room temperature and 990 nm at 6 K. In contrast to man y other species, the reaction center showed two well-separated absorption b ands of bacteriopheophytin at 6 K, located at 747 and 762 nm. The primary e lectron donor showed a bleaching band centered at 925 nm upon phstooxidatio n. Thus, the energy gap between LH1 and the primary electron donor is quite large in this strain: 425 cm(-1). Nevertheless, trapping occurred with a t ime constant of 65 +/- 5 ps, similar to the rates observed in other purple bacteria. As in other species, no back-transfer from the reaction center to the antenna was observed. Our results show that strain 970 is a unique sub ject for the study of antenna and reaction center function and organization .